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Irritrol 700 valve will not close

Hi guys.
With your help, I designed and installed an irrigation system with 14 zones back in 2006. The whole system is Hunter except for 4 zones that are drip lines in the flower beds. I'm having a problem in one of the flower beds.
The problem is that one zone comes on and stays on. The zone will keep dripping until I remove the plug on the bottom of the filter screen housing to relieve the water pressure.
Zonehas a Irritrol 3/4 inch 700 series valve leading to a filter screen that looks like this. http://www.sprinklerwarehouse.com/DIG-Drip-Irrigation-Filter-Screen-p/p10-155.htm It is a black plastic housing with a screen inside. Then the flow is to a blue stripe hose with built in emmitters.
I just replaced the solenoid and it has not solved my problem.
Also have removed the diapram, spring and the wedge shaped seat float. Is there a way to remove the white disc under the solenoid and clean?
What do you think? Since it comes on I doubt it is a power supply prob but I have expansion spaces in the control box. Diaphram looks OK. I can swap diapram with another zones controller.
Thanks!

OK - swapped out the diaphrams and that is the problem.
I have a question about the best way to reassemble the 3/4" Irritrol 700. It seems difficult to get the float stop set on the spring and I wonder if that may be a contributing factor. Also is it best to put the diaphram on the cap and assemble or should I put the diaphram on the valve body and then position the cap.
Here is what I did after cleaning and inspecting:
1. Set spring in place
2. Position detent in metal arm of float stop on top of spring and press down on float.
3. Set black circular retaining piece on top of float stop with the small detent box facing up towards diaphram.
4. Position diaphram on valve body.
5. Set valve cap on top of diaphram.
6. Screw single phillips screw into top of valve cap
7. Position 4 machine bolts thru cap and screw into valve body.
8. Tighten all screws
9. Screw in solenoid
10. turn on water supply and ck for leaks

That's a very tricky valve to rebuild. Not a lot of non pros could do it. Good job. You definetly want to put the diaphragm on the body not the bonnet.

You asked about the white disc on the solenoid. Just grab it with something and pull it off. Throw it away it doesn't do anything but hold the plunger in place so you don't lose it. I've seen that piece keep a valve from shutting off on a few occasions.

I believe the white solenoid "keeper" ring would be a possible suspect when a zone valve has problems opening fully, or opening at all. Not an issue on brand-new valves, but when solenoids are replaced on older valves.

It is a difficult valve to repair. The repair kit cost more than a complete valve. Opening the valve on a work bench made it easier to see how to properly rebuild.

Part of my problem was getting the float to set properly on the spring and then keep it in place while fitting the retainer piece (black ring piece). The stainless part of the float needs to sit in a notched recess just below the spring in the lower valve housing. Hold it in place with a screwdriver tip while positioning the retainer ring over the float. That is the key.

Thanks for the tip on placing the diaphram on the lower valve before installing the bonnet. The only was to make sure the diaphram seats ovet the male nubs at the top and bottom of the lower valve body.